Heretofore, a typical example of an epoxy resin composition suitable for impregnation was a bisphenol type epoxy resin containing an acid anhydride, e.g. methyltetrahydrophthalic anhydride, as a curing agent and a tertiary amine, imidazole or metal chelate compound as an accelerator. This conventional composition generally has a short pot life and a viscosity incompatible with workability, i.e. the viscosity is too high. In order to improve workability (to lower the viscosity), monoglycidyl ethers such as butyl glycidyl ether, allyl glycidyl ether and the like, or diglycidyl ethers such as ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether and the like, can be used. However, there is the disadvantage that an improvement in pot life can not be expected.
One of the easiest methods to overcome the above mentioned disadvantage is to add a curing agent and an accelerator to the epoxy resin at the time of using the resinous composition. However, this method is not employed to apply insulation to the coils of electrical machinery and apparatuses on an industrial scale since the operation is complicated and work efficiency is very low.
Accordingly, the above mentioned epoxy resin is usually stored in a resin tank, and is kept cool to prolong the pot life. However, even by this method, the pot life is limited and the viscosity increases. Therefore, a part of the resinous composition must be disposed of and a newly prepared resinous composition must be supplied to modify the viscosity. Thus, the disposal and the supply must be repeated. This is a very unfavorable point in view of industrial and economical considerations.
Under these circumstances, it is presently required that an epoxy resin composition having not only good electrical and mechanical properties but also excellent workability and a long pot life be provided.